1. Field of the invention
The invention concerns telecommunication by means of electromagnetic waves and in particular microwave amplifier devices used in transmit and/or receive equipment in such telecommunication systems. To be more precise, the invention concerns a predistortion linearizer for microwave amplifiers intended to alleviate the effects of non-linearity due to different amplification conditions in the various operating regimes of the equipment.
2. Description of the prior art
The person skilled in the art knows that to operate efficiently an amplifier, especially a power amplifier, must operate near saturation, and hence the need for a linearizer. Close to saturation the linearity of the amplifier is strongly degraded compared to the linearity of the same amplifier used with signals of lower amplitude than those required for operation under saturated conditions.
A non-linearity correction device known as a linearizer can be used to increase the linear dynamic range of an amplifier without compromising the electrical efficiency achieved near saturation. There are three types of linearizers: the feedback type, the feed forward type and the predistortion type.
The feedback type linearizer is suitable for amplifiers operating at relatively low frequencies in which variations in the signal characteristics (and therefore in the linearity of the amplifier) are slower than the loop response time, i.e. the time between detection of a non-linearity and its correction. On the other hand, this configuration is very difficult to use for amplifying microwave signals, as it is too slow (compared to the signal frequency) for results to be satisfactory.
Thus for microwave signals it is preferable to use either a feed forward linearizer or a predistortion linearizer. The principle of operation is based on the extraction by a coupler of part of a signal before amplification which is then processed by various active and passive electronic components to produce a non-linear correction signal having the same non-linearity characteristics as the wanted signal to be corrected, but with the opposite phase. In a feed forward linearizer circuit adding this correction signal to the amplified wanted signal achieves the required linearity. On the other hand, a predistortion linearizer supplies a correction signal with the signal to be amplified at the input of the microwave power amplifier the non-linearity of which is to be corrected.
Various types of predistortion linearizer have already been designed. A first family of predistortion linearizers is described in U.S. Pat. No. 4,992,754, BLAUVELT et al., assigned to ORTEL Corp. (USA), for example. According to this document, the wanted signal is applied to a delay line and the correction signal is produced in a parallel branch of the circuit with the amplitude adjusted so that it is equal to that of the non-linearity of the amplified signal, the phase of the signal being varied so that it is in phase opposition to the wanted signal in the delay line. The wanted signal and the correction signal are added by means of a microwave coupler (power combiner) and are then fed to the input of the microwave power amplifier. The non-linearities at the output of the power amplifier stage are significantly reduced or even eliminated by this means.
Another type of predistortion linearizer is known from U.S. Pat. No. 4,068,186 SATO et al., assigned to KDD (Japan). This linearizer, shown schematically in FIG. 1, is designed to operate at high frequencies and to alleviate the non-linearity of a klystron or TWT (travelling wave tube) type amplifier. SATO et al. teach the use of a low-power TWT as a non-linearity generator. This type of amplifier introduces a slight time-delay of the amplified signal relative to the unamplified signal, due to the finite propagation speed of electrons in a vacuum. It is therefore necessary to include a delay line 3 in the corrector branch of the circuit to synchronize the non-linear and linear channel signals.
The signal to be amplified is applied to the input 1 of the SATO et al. linearizer, which is connected to a first amplitude-frequency characteristic corrector 10 the output of which is fed to a coupler (power divider) 2 which splits the signal into two parts fed to the inputs of respective transmission lines, a first of which includes a delay line 3 and the second of which includes a microwave amplifier 4 generating non-linear distortion, for example a low-power TWT. The fourth branch of the coupler 2 is terminated by a matched load 8. The coupler 2 conventionally introduces a phase shift .theta. between the two output signals. The coupler can be a 3 dB hybrid coupler, for example, in which case the phase shift is .theta.=.pi./2=90.degree..
The main channel (or linear channel) includes a microwave amplifier 4 the operating point of which is chosen near saturation to generate non-linearities which depend on the power of the input signal. A variable attenuator 13 is provided at the output of the amplifier 4 so that the output level of the predistortion linearizer device can be varied without altering the gain of the amplifier 4.
In the SATO et al. disclosure the other (nonlinear) channel includes a phase-frequency characteristic corrector 9 in addition to the delay line 3. The signals from the two channels are applied to two inputs of a coupler (power combiner) 5 which adds them together (still with a phase shift .theta. between the two signals). This coupler can be a 3 dB hybrid coupler, for example, which introduces a further phase shift .theta.=.pi./2. The fourth branch of the coupler 5 is terminated by a matched load 7. The signal obtained by adding the two signals at the two inputs is then fed to a second amplitude-frequency characteristic corrector 12. The wanted signal, complete with the predistortion, is then fed to the output 6 of the device from which it is fed to the input of a microwave power amplifier.
This prior art device therefore includes two nonlinear amplitude-frequency characteristic correctors 12 and 10 plus a phase-frequency characteristic corrector 9. Embodiments of such corrector units are described in the SATO et al. document, the disclosure of which is hereby incorporated by way of reference, constituting description of the prior art.
The non-linear characteristics of these units are added to those of the microwave amplifier 4. The transfer functions of all these units vary in a disparate manner with the signal frequency and amplitude and with the temperature of the components. The non-linear signal produced in this way is added to the wanted signal with the opposite phase to cancel the non-linearity of the microwave power amplifier (not shown), but cancellation is obtained only in relatively narrow frequency band, over a relatively narrow range of input power and at a given temperature.
A transfer function with gain and phase increasing with the input signal level can be obtained by varying the characteristics of the corrector units 9, 10, 12. This is usually the required response for linearizing a power TWT.
A transfer function with the gain increasing with the input signal level but the phase decreasing with the latter can be obtained by a different adjustment of the corrector units 9, 10, 12. This is the response required to linearize a solid state power amplifier.
Another broadband microwave linearizer is described in the article by A. M. KHILLA "Novel broadband linearizers and their application in power amplifiers for satellite transponders and ground stations", published in Proceedings Second European Conference on Satellite Communications, Liege, Belgium, 22-24 Oct. 1991, pp 229-234, published by ESA (European Space Agency), publication No SP-332, the content of which is hereby incorporated by way of reference, constituting a description of the prior art.
This document teaches the use of a predistortion circuit for linearizing a broadband amplifier of a Ku band satellite transponder. As in the SATO et al. document, the circuit has two branches connected at their ends by two 3 dB hybrid couplers, each introducing a phase-shift of 90.degree.. It further teaches the use of equal electric lengths in the two branches, a configuration that is relatively rare in other predistortion circuits described in the literature.
The operating principle of predistortion linearizers as described in the prior art documents cited hereinabove can be used in microwave applications. Nevertheless, it has a number of major drawbacks which make it difficult to use.
A first drawback is inherent in the fact that the initial adjustments of the amplitude-frequency characteristic corrector circuits (10, 12) and the phase-frequency characteristic corrector circuit (9) are often difficult and time-consuming, and constitute a task which is even more complicated in the usual case in which the electrical lengths of the two channels are very different.
A second drawback results from the fact that the two channels comprise components of different kinds, with different responses to temperature variations; this causes variations in the characteristics of the corrector units and therefore in the overall transfer function of the device, these variations being highly temperature-sensitive.
A further drawback of the prior art devices is that the optimum bandwidth of the non-linearity corrector device is limited by the differences between or by different variations in the electrical lengths of the two channels, depending either on the frequency or the amplitude of the signals to be amplified or on the operating conditions and in particular the temperature.
An object of the invention is to alleviate the drawbacks of the prior art.